The field to which the present invention pertains is concerned with floating-type offshore structures, broadly. More particularly, the present invention relates to a floating-type offshore structure adapted for use in an icy sea area in particular as well as in iceless offshore areas.
Conventionally, in operating for example an oil-well drilling in ice-covered or otherwise icy areas, it has in most cases been practised to form a sort of island at the predetermined surface site for the well drilling and, installing the drilling machinery and equipment on such artificially formed island, carry out drilling operations with the so installed machinery. A difficulty with such conventional art consists in that the artificial island cannot with ease be transferred from a first drilling site to a second one. Another difficulty is that if the water depth is so great as to exceed 30 m for example, the formation of an artificial island itself can hardly be performed.
Then, for comparable offshore operations in iceless sea areas, use is made of a self-propelled floating-type rig, a jack-up type rig or a semi-submersible rig. Inconveniently, however, these rigs are not so structured as to well stand pressures of drift ice, and their usefulness in an icy offshore area is therefore confined in summer seasons or under an iceless condition of that sea area.
In the case of semi-submersible rigs for example, they individually comprise a platform, which is supported suitably above the sea level by a plurality of struts mounted to stand on a lower hull so that the platform is not subject to influence of wave motion. If such a rig is put for use in an icy seawater area or under an icy seawater condition, however, its struts are subjected to pressures of drift ice, wherefore it cannot be stably moored by means of normally employed wire ropes.
Also, the lower hull of the rig under consideration comprises a twin hull structure, and in case the twin hull structure comprises a series of struts mounted on each of the two hull members, pressure of drift ice which the rig will undergo can widely vary depending upon various directions in which the drift ice moves. Thus, with the rig in this case, a stable position control thereof can hardly be obtained.
In the known rig structures, further, the drilling pipe extended from the platform to the sea bed is exposed to drift ice and is therefore prone to damages by floating ice pieces colliding against the same.
Accordingly, a primary object of the present invention is to eliminate the above indicated and other difficulties with the present art, and to that end, make buoyant or floating-type offshore structures employable or useful in not only iceless sea areas or iceless conditions of the sea but also ice-covered or icy sea areas or icy conditions of the sea.
Another object of the invention, which is based on the knowledge that the resistance of ice plates or floes against a bending stress is relatively low, is to cause to take place a downward flexural failure of drift ice sheet and thereby realize a minimization of the effect of pressure application by the drift ice sheet on the offshore structures.
Still another object of the invention is to minimize different influences of the pressure application by drift ice sheets at different points of or in different positions of the offshore structure which are likely depending upon a change in the direction in which the drift ice sheets move relative to the structure.
To attain these and other objects, the present invention provides a floating-type offshore structure the main body of which comprises a lower hull, a plurality of struts mounted to stand on the lower hull and a platform supported above the seawater surface by the plurality of struts and which is characterized by being provided with a ballast tank or tanks formed in the lower hull and/or the struts and also by the struts which have downwardly converging faces accepting contact by drift ice pieces or floes.
According to the present invention, the ice contacting face of the strut is downwardly converged as mentioned above, therefore it can cause an ice floe in contact with the inclined face of the strut to more easily undergo bending as it is increasingly pushed toward the strut by its following floe or floes to eventually undergo downward flexural failure with the result that the pressure the ice floe applies to the offshore structure is minimized, making the structure useful in an ice-covered condition of an ocean area.
Also, by suitably adjusting the amount of water received in the ballast tank or tanks and thereby adjusting the draft plane of the floating-type structure so as to adapt the structure for use in a semi-submerged state, it is feasible in accordance with the present invention to minimize the influence of wave motion upon the floating structure in summer seasons or under iceless conditions of the ocean.
Moreover, it is practicable according to the invention to dispose the plurality of struts in a ring arrangement surrounding a drilling pipe device so as to provide a protective means for the latter. It may further be devised to make the platform rotatable about a strut and thereby realize a minimization of the influence of ice floes upon the structure which varies depending upon variable flow directions in which the ice floes move.
The foregoing mentioned and other objects, features and advantages of the present invention will more clearly appear from considering the following description taken in conjunction with the accompanying drawings.